stdlib: Remove final decorator from abstract board
stdlib: Remove final decorator from abstract board

@typing.final was not introduced until python3.8.

Change-Id: I65a5f4b5655d2ad80f7b566f9cea8e2371b3be62
Signed-off-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55863
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56246

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